Light emitting diodes (LEDs) are used in vehicles to indicate whether switchable functions are on or off and also to backlight switches that switch the function on and off. In many cases, a switch that is used to switch a function on and off is packaged with a LED that is on and thus illuminated when the function has been switched on and off and thus not illuminated when the function has been switched off. This LED is often referred to as an indicator jewel and will be referred to herein as an “indicator LED.” Also in many cases, such a switch is also packaged with a backlight LED that is turned on, such as in low ambient light conditions such as nighttime, to backlight the switch so that a vehicle driver is better able to see it such as in low ambient light conditions. This LED will be referred to herein as a “backlight LED.” In many cases, such a backlight LED is turned on when the vehicle's lights (are turned on and turned off when the vehicle's lights are turned off. In this regard, a vehicle's parking lights can be turned on without turning the vehicle's headlamps and taillights on and remain on when the vehicle's headlamps and taillights are turned on.
As is known, indicator LEDs are typically at a brighter level during high ambient light conditions, such as daylight, and often at a dimmer level during low ambient light conditions, such as nighttime. In contrast, backlight LEDs are typically off, or driven at a dimmer level, during high ambient light conditions and at a brighter level during low ambient light conditions.
FIG. 1 is a simplified schematic of a typical circuit used in vehicles to illuminate indicator LEDs and backlight LEDS. FIG. 1 shows an indicator switch 100 having a switch 102, an indicator LED 104 and a backlight LED 106. Indicator LED 104 is coupled to an output 108 of an electric control unit 110 of a vehicle, such as a body control module of the vehicle, at which a pulse width modulated output signal is provided. Output 108 will be referred to herein as indicator LED PWM output 108. Backlight LED 106 is coupled to an output 112 of electronic control unit 110 at which a pulse width modulated output signal is also provided. Output 112 will be referred to herein as backlight LED PWM output 112. In operation, ECU 110 sets the duty cycle of the pulse width modulated signals at indicator LED PWM output 108 and backlight LED PWM output 112 to drive the indicator LED 104 and backlight LED 106 to achieve the desired illumination. When backlighting is not on, for example when the vehicle lights are off, ECU 110 sets the duty cycle of the pulse width modulated output signal at indicator LED PWM output 108 at a high duty cycle so that indicator LED 100 outputs a high light level. ECU 110 also either doesn't output a pulse width modulated signal at backlight LED PWM output 112 so that backlight LED 106 is off or sets the duty cycle of the pulse width modulated output signal at backlight LED PWM output 112 at a low level so that backlight LED 106 outputs light at low level. When backlighting is on, for example when the vehicle lights are on, ECU 110 sets the duty cycle of the pulse width modulated signal at indicator LED PWM output 108 at a lower duty cycle to that LED 100 outputs light at a lower level than during high ambient light conditions. ECU 110 also sets the duty cycle of the pulse width modulated signal at backlight LED PWM output 112 at a duty cycle so that backlight LED 106 outputs light at a desired level that is higher than during high ambient light conditions. It should be understood, that the light intensity level of both indicator LEDs and backlight LEDs are typically user adjustable such as by the adjustment of a potentiometer or other device (not shown) coupled to ECU 110. ECU 110 responds to the user adjustment and sets the duty cycles of the PWM signals output at indicator LED PWM output 108 and backlight LED PWM output 112 accordingly.
In some cases, the indicator LED of an indicator switch for a vehicle function is not driven by a PWM output of an ECU although the backlight LED is. Rather, it is simply switched on or off by a function control module that controls that vehicle function. For example and with reference to FIG. 2, one type of function control module is electronic parking brake control module 208 that directly switches on and off the indicator LED 204 of an indicator switch 200 having a switch 202 used to switch the electronic parking brake (not shown) on and off. In this example, an anode 212 of indicator LED 204 is coupled to a source of DC voltage, such as 12 VDC, and a cathode 214 of indicator LED 204 is coupled to a LED drive output 210 of electronic parking brake control module 208. Electronic parking brake control module 208 couples the cathode of indicator LED 204 to common to turn indicator LED 204 on and decouples the cathode of indicator LED 204 from common to turn indicator LED 204 off. A backlight LED 206 of this indicator switch 200 is coupled to the backlight LED PWM output 112 of the ECU 110 and remains controlled by ECU 110 in the same manner as backlight LED 106 is controlled by ECU 110. In this type of indicator switch, when the indicator LED 204 is on, it is always on at the same light level and can't be dimmed. The indicator LED 204 is thus on at full brightness whether backlighting is on or off. As is known, however, it is desirable that indicator LEDs when they are on be brighter during when backlighting is off (such as during daytime when the lights of the vehicle are off) and dimmer when backlighting is on (such as during nighttime when the lights of the vehicle are on). It should be understood that the function control module can be for controlling functions other than an electronic parking brake and that references to electronic park brake module 208 should be understood to refer to any function control module that controls the indicator LED of an indicator switch in the described manner.